The invention relates to an image projection system comprising, in this order, an illumination system, an image display system with at least one reflective image display panel for modulating, an illumination beam to be supplied by the illumination system with image information, and a projection lens system.
An image projection system of the type described in the opening paragraph is known, for example, from U.S. Pat. No. 5,905,545. The image projection system described in this specification comprises an illumination system for supplying an illumination beam, and an image display system with reflective image display panels for modulating this light beam in conformity with image information to be projected. The display panel may be, for example, a digital micro-mirrored display (DMD) with a two-dimensional array of reflective digital light switches which are driven by means of electrodes. Small mirrors for each pixel are used to switch a pixel on or off by changing an angle of the mirrors. The pixels of the DMD can maintain their xe2x80x98onxe2x80x99 or xe2x80x98offxe2x80x99-state for controlled display times. To achieve intermediate levels of illumination between white and black, pulse-width modulation techniques are used. In the reflective DMD projection systems, the part of the light modulated by the display panel, which part must give rise to dark pixels in the image, is deflected from the light path by the reflective switches and absorbed in the optical system and is thus lost. This is at the expense of the peak brightness in the image.
It is an object of the present invention to provide an image projection system in which a relatively high peak brightness is realized.
This object is achieved by the image projection system according to the invention, which is characterized in that the illumination system comprises an extra optical system for at least partly re-illuminating the image display system with the light reflected by the image display system to the illumination system, the extra optical system comprising means for redistributing light coming from a pixel of the image display system across a plurality of pixels of the image display system.
The present invention relates to a reflective projection system in which light is incident on the display panel and modulated by the display panel before it is projected. The invention is based on the recognition that the light which is modulated by a pixel representing a dark or grey pixel in the image is deflected from the light path but is not absorbed in the display system and is again sent towards the entrance of the optical display and thus recuperated. This light will as yet have an opportunity of being incident on a pixel representing a bright pixel. In the image projection system described above, the light intended for dark or grey pixels is thus not lost but is re-used. Furthermore, to prevent ghost images from being produced during this reuse, the illumination system is provided with means for redistributing of the light.
A preferred embodiment of the image projection system according to the invention is characterized in that the extra optical system comprises a lens element and an optical fiber, the lens element being situated between an element of the reflective image display panel and an input of the optical fiber for concentrating reflected light in the optical fiber. In this way, the reflected light which is not used to form a projection image can be efficiently transported back to the illumination system. Total internal reflection in the optical fiber redistributes the recuperated light.
A further embodiment of the image projection system according to the invention is characterized in that the extra optical system comprises an optical wedge for combining the light from the radiation source and the reflected light from the reflective image display panel.
A further embodiment of the image projection system according to the invention is characterized in that the extra optical system comprises an integrating rod for receiving light from an element of the reflective image display panel, and reflecting means situated at one side of the integrating rod for reflecting the received light back to the image display panel. In this way, the integrating rod acts as a non-imaging mirror device which homogeneously distributes the light on the image display device so that small distortions are reduced in the projected picture.
A further embodiment of the image projection system according to the invention is characterized in that the image projection system comprises a further reflective image display panel for modulating a second beam provided by the illumination system, and the extra optical system comprises means for recombining the light reflected from both the reflective image display panel and the further reflective image display panel. For example, a dichroic mirror may be used to recombine the light of different colors from the respective reflective image display panels into a single light beam which can be directed to the illumination system.